Topic Contents

Thyroid Hormone Tests

Test Overview

Thyroid hormone tests are blood tests that check how well the
thyroid gland is working. The thyroid gland makes
hormones that regulate the way the body uses energy.

The thyroid
gland is a butterfly-shaped gland that lies in front of your windpipe
(trachea), just below your voice box (larynx). The thyroid gland uses iodine
from food to make two thyroid
hormones: thyroxine (T4) and
triiodothyronine (T3). The thyroid gland stores these
thyroid hormones and releases them as they are needed.

Thyroid
hormones are needed for normal development of the brain, especially during the
first 3 years of life.
Intellectual disability may occur if a baby's thyroid gland
does not produce enough thyroid hormone (congenital
hypothyroidism). Older children also need thyroid
hormones to grow and develop normally, and adults need the hormones to regulate
the way the body uses energy (metabolism). The
United States Preventive Services Task Force
recommends that all newborns be tested for congenital hypothyroidism.footnote 1

Thyroid hormone blood tests include:

Total thyroxine (T4). Most of the thyroxine (T4)
in the blood is bound to a protein called thyroxine-binding globulin.
Less than 1% of the T4 is free. A total T4 blood test measures both free and bound
thyroxine. Free thyroxine affects tissue function in the body,
but bound thyroxine does not.

Free thyroxine (FTI or FT4). Free thyroxine (T4)
can be measured directly (FT4) or calculated as the free thyroxine index (FTI).
The FTI tells how much free T4 is present compared to bound T4. The FTI can help tell if abnormal amounts of T4 are present because
of abnormal amounts of thyroxine-binding globulin.

Triiodothyronine (T3). Most of the T3 in the blood
is attached to thyroxine-binding globulin. Less than 1% of the T3 is
unattached. A T3 blood test measures both bound and free triiodothyronine. T3
has a greater effect on the way the body uses energy than T4, even though T3 is
normally present in smaller amounts than T4.

Why It Is Done

Thyroid hormone tests are done
to:

Find out what is causing an abnormal
thyroid-stimulating hormone (TSH) test. For more information, see the topic
Thyroid-Stimulating Hormone (TSH). This is the most
common reason for thyroid hormone tests.

Check how well treatment
of thyroid disease is working. The total thyroxine (T4), free thyroxine (FT4),
and free thyroxine index (FTI) values are often used to keep track of treatment
for
hyperthyroidism.

Screen newborns to find
out if the thyroid gland function is normal. A condition called congenital
hypothyroidism can prevent normal growth and
development and cause other severe problems, such as
intellectual disability, if it is not treated soon after
birth.

How To Prepare

Many medicines may change the results
of this test. Be sure to tell your doctor about all the nonprescription and
prescription medicines you take. If you are taking thyroid medicines, tell your
doctor when you took your last dose. Your doctor may instruct you to stop
taking thyroid medicines temporarily before having this test.

Talk
to your doctor about any concerns you have regarding the need for the test, its
risks, how it will be done, or what the results will mean. To help you
understand the importance of this test, fill out the
medical test information form(What is a PDF document?).

How It Is Done

Blood test

The health professional taking a sample
of your blood will:

Wrap an elastic band around your upper arm to
stop the flow of blood. This makes the veins below the band larger so it is
easier to put a needle into the vein.

Clean the needle site with
alcohol.

Put the needle into the vein. More than one needle stick
may be needed.

Attach a tube to the needle to fill it with
blood.

Remove the band from your arm when enough blood is
collected.

Put a gauze pad or cotton ball over the needle site as
the needle is removed.

Put pressure on the site and then put on a
bandage.

Heel stick

A heel stick is used to obtain a blood
sample from a newborn. The baby's heel is pricked with a sharp instrument
(lancet) and several drops of blood are collected.

How It Feels

Blood test

The blood sample is taken from a vein
in your arm. An elastic band is wrapped around your upper arm. It may feel
tight. You may feel nothing at all from the needle, or you may feel a quick
sting or pinch.

Heel stick

A brief pain, like a sting or a pinch,
is usually felt when the lancet punctures the skin. Your baby may feel a little
discomfort with the skin puncture.

Risks

Blood test

There is very little chance of a
problem from having blood sample taken from a vein.

You may get a small bruise at the site. You
can lower the chance of bruising by keeping pressure on the site for several
minutes.

In rare cases, the vein may become swollen after the blood
sample is taken. This problem is called phlebitis. A warm compress can be used
several times a day to treat this.

Heel stick

There is very little chance of a
problem from a heel stick. A small bruise may develop at the site.

Results

Thyroid hormone tests are blood tests
that check how well the
thyroid gland is working.

Normal

The normal values listed here—called a reference range—are just a guide. These ranges vary from lab to lab, and your lab may have a different range for what's normal. Your lab report should contain the range your lab uses. Also, your doctor will evaluate your results based on your health and other factors. This means that a value that falls outside the normal values listed here may still be normal for you or your lab.

Results are usually available within a few days.

Labs generally
measure free T4 (FT4) levels, but also may measure total thyroxine (T4) and T3
uptake (T3U). Results of these thyroid hormone tests may be compared to your
thyroid-stimulating hormone (TSH) results.

What To Think About

Because
false-positive results can occur when testing a
newborn for congenital hypothyroidism, the thyroid hormone tests may be
repeated a few days after initial testing. If the results are still abnormal
and congenital hypothyroidism is suspected, additional testing is done.

Other blood tests are often used to check how
well the thyroid gland is working.

Thyroid-stimulating hormone (TSH) test measures the amount of TSH in the blood and is considered the
most reliable way to find a thyroid problem. If the TSH test is abnormal, other
thyroid hormone tests such as a T3 or T4 may be done. For more information, see
the topic
Thyroid-Stimulating Hormone.

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